1. Field of the Invention
The present invention relates to a battery charger for charging an external battery being detachably mounted to the charger, and particularly to a battery charger for charging the external battery by means of a lithium-ion secondary battery incorporated in the charger along with electric power from a commercial power supply or the like.
2. Description of the Related Art
The assignee of the present invention has previously developed a battery charger having a reserve battery incorporated for the purpose of shortening an involved charging time. (Refer to Japanese Laid-Open Patent Publication No. 2003-199260).
FIG. 1 shows a circuit diagram of a battery charger as disclosed in Japanese Laid-Open Patent Publication No. 2003-199260. The battery charger includes: an electrically charging source 92 for charging an external secondary battery 91; a main charge switch 94, interconnected to and between the charging source 92 and the external battery 91, for switching on and off the external batteries 91 to be charged in a mutually delayed timing of pulse charge; a reserve battery 96, connected through an auxiliary charge switch 95 to the charging source 92, for being charged by the charging source 92; a sub charge switch 97 connected to and between the reserve battery 96 and the external battery 91; and a control circuit 93 for controlling the switching on and off the sub charge switch 97, the main charge switch 94 and the auxiliary charge switch 95. The control circuit 93 switches on and off the main charge switch 94 to sequentially pulse-charge a plurality of external batteries 91, and also switches on the auxiliary charge switch 95 at a timing when the charging source 92 does not charge the external battery 91, so that the charging source 92 controls the charging of the reserve battery 96. Further, the control circuit 93 sequentially switches on and off the main charge switch 94, and in the pulse-charging timing that the external battery 91 is pulse-charged, the main charge switch 94 is switched off, and the main charge switch 94 is switched off to switch on the sub charge switch 97 connected to the external battery 91, which is not pulse-charged by the charging source 92, so that the external battery 91 is charged by means of the charging source 92 as well as the reserve battery 96.
The battery charger is able to charge a secondary battery within a shorter time period without increasing the amount of pulsed current for a pulse charge. This is because when a charging current is not supplied to the secondary battery from the charging source, or when a charging current smaller than set currents is supplied from the charging source to the secondary battery, the charging source supplies the charging current to an auxiliary battery to fully charge the auxiliary battery, so that the secondary battery is charged in a delayed timing both from the auxiliary charge current supplied from the charged reserve battery and from the pulsed-charging current supplied from the charging source. In particular, the battery charger is able to charge the external battery effectively within a shorter period of time, by delaying the timing of charging from the pulse charge and from the reserve battery, in other words, without increasing the amount of pulsed current for the pulse charge by charging the reserve battery in a timing of not pulse-charging. Since the battery charger is able to charge the external battery within a shorter period of time without increasing the amount of pulsed current for the pulse charge, a quick recharge becomes possible without increasing the output current from the charging source.
The assignee of the present invention has fabricated the battery charger as a charger for a nickel-hydrogen battery, a nickel-cadmium battery, etc. In the battery charger, the internal reserve battery is made of the same type as the external battery. This is because the reserve battery is connected via a diode to the external battery so that the external battery is charged from the reserve battery. Since the reserve battery and the external battery are of the same type in the battery charger, the external battery can be charged by the reserve battery connected to the external battery, without the external battery being subjected to an overcharge. However, the battery charger uses an internal reserve battery for the purpose of shortening the charging time needed for the external battery to be charged, that is to say, in order to increase an average current amount from the charging current. Further, an electric source for all the circuits in controlling the charge of the external battery is supplied by input power from a commercial power supply. As such, when the commercial power supply fails or becomes unavailable, the control circuit becomes inoperable, resulting in failure to charge the external battery by means of the reserve battery. Therefore, when a power plug is unplugged from the battery charger, the charge on the external battery comes to stop as well. For example, it may happen that the external battery during a charging operation is carried around when the battery is not yet in a fully charged state. In such a state as the power plug is unplugged, a conventional type of battery charger is unable to charge the external battery even when the reserve battery is already in a fully charged state.
The present invention has thus been made to solve the above-mentioned difficulties. It is the primary object of the invention to provide a battery charger which is able to charge continuously by means of an internal lithium-ion secondary battery, like when the power plug is unplugged to interrupt input power even while the external battery is not yet in a fully charged state.
It is another object of the invention to provide a battery charger, with a lithium-ion secondary battery being built inside, which is able to effectively charge the external battery by means of the internal lithium-ion secondary battery, even when the battery charger has not been used for a long period of time.
Further, another major object of the invention is to provide a battery charger, with a lithium-ion secondary battery being built inside, so that the internal battery is constantly kept in a fully charged state, and thus the external battery can be efficiently charged.